It has been known that a number of endogenously produced mammalian humoral factors, i.e. cytokines and hemopoietic factors possess important biological activities that are considerably helpful in treating various human diseases.sup.1,2. Many of these factors are being tested in man, those with proven efficiency being commercially available as medicinal agents.
The following cytokines and hemopoietic factors are being most extensively researched in oncology: interleukin 2 (IL-2).sup.3,4, tumor necrosis factor alpha (TNF-.alpha.).sup.5, erythropoietin, macrophage-granulocyte and granulocyte colony-stimulating factors (GM-CSF and G-CSF, respectively.sup.6,7). No less actively is being studied the use of cytokines and hemopoietic factors for the treatment of infectious disease: interferons (IFN-.gamma. and IFN-.beta.).sup.8,9,10, colony-stimulating factors.sup.11,12, and the like.sup.13. Colony-stimulating factors and erythropoietin are broadly used in hematology.sup.14,15.
However, the medicinal use of these exogenously administered agents has its limitations associated with the lack of acceptable drug formulations or their exorbitant cost, a short half-life of these substances in biological media, difficulties in dose finding as well as numerous toxic and allergic effects.sup.16,17, since even the recombinant products are more or less immunogenic to the human organism because of the processing fluctuations in the course of the artificial synthesis.
In this regard, in view of achieving a more invariable and significant therapeutic effect free of adverse reactions, it is preferable to induce the endogenous production of the autologous cytokines and hemopoietic factors immediately within the organism of a subject. The remedial effect due to such intrinsic stimulation is free of all the disadvantages associated with exogenously introduced cytokines and hemopoietic factors.
A number of compounds are currently being evaluated that stimulate endogenous production of cytokines and hemopoietic factors in both experimental and clinical settings. There are universally known cases, including successful ones, of using microbial products for cancer therapy which in recent decades has been shown to be mediated via stimulation of the tumor necrosis factor endogenous production.sup.18. The products capable of evoking concomitant production of various cytokines and hemopoietic factors have presently come to be known as multi-cytokine inducers. Among these are a killed streptococcal preparation, Nocardia Opaca, and other bacterial products.sup.19,20,21. However, virtually all the substances possessing such capability are either killed microorganisms or microbial products or compounds having irregular composition, which results in their limited medicinal utility or even renders their therapeutic use impracticable. Thus, the problem of finding a medically and pharmaceutically acceptable inducer of the cytokine and hemopoietic factor endogenous production has not heretofore been resolved.
Oxidized glutathione (also known as glutathione disulfide and GSSG) will often be referred to as GSSG in this application.
GSSG is known as a dimmer of tripeptide glutathione (.gamma.-glutamyl-cysteinyl-glycine) where two molecules of the tripeptide with the above structure are linked via a covalent disulfide bond between the cystamine residues. Therefore, both the tripeptide glutathione (glutathione, reduced glutathione, GSH; hereinafter referred to as GSH) and its dimmer GSSG are natural metabolites present in animal tissues and biological fluids. At the same time, the natural blood level of GSSG is not sufficient for inducing the cytokine endogenous production in both normal and pathological conditions.
GSH is known to be one of the most important intermediates in the amino acid metabolism and a factor maintaining the intracellular homeostasis.sup.22,23. The reducing properties of GSH and its function as a donor of reduction equivalents, which is due to the sulfhydryl moiety of the cystamine residue, are of key importance. This characteristic of GSH is responsible for the substance playing a crucial part in one of the most important intracellular antioxidant systems, consisting of GSH as such and two enzymes of its reversible conversion into GSSG: glutathione peroxidase and glutathione reductase.sup.24,25. The permanent functioning of said system is essential for inactivating or reducing endogenously generated oxidants as well as active metabolites of foreign substances.sup.26,27.
GSH is also known to participate in detoxification reactions involving a group of enzymes collectively known as glutathione S-transferase.sup.28. These enzymes are capable of conjugating the GSH molecule with various xenobiotics by forming a bond between the latter and glutathione via the thiol moiety of the cystamine residue of the tripeptide. The subsequent degradation of the conjugate is catalyzed by the .gamma.-glutamyl cycle enzymes, and may vary considerably depending upon the nature of the xenobiotic.
Under natural conditions, GSSG does not accumulate in amounts sufficient for inducing cytokine and hemopoietic factor production, due to a constant reduction of GSSG to GSH. The GSSG reduction to GSH also actively progresses in the intestines and liver upon GSSG oral administration, and as any product made of amino acids, the substance is proteolytically degradable in the gastrointestinal tract.
GSSG is known to be used as a components of a nutritional supplement utilized as an adjunct diet in treating patients.sup.29. However, being a peptide substance, most of the orally administered GSSG is digested in the gastrointestinal tract with the remainder being reduced in the intestinal and hepatic cells to GSH and not entering the circulation. Therefore, the delivery of GSSG into the organism through the gastrointestinal tract may eliminate the possibility of the realization of its activity as a stimulator of endogenous production of cytokines and hemopoietic factors.
An elevation of the GSH endogenous levels for medicinal utility is known to be suggested for boosting immunity.sup.30 and treating toxemias, poisonings, diabetes, mellitus, cardiovascular, infectious and other disorders.sup.31,32,33. Possible functions of GSH and GSSG appear in the literature.
Exogenous GSH or its direct (.gamma.-glutamyl-cystamine, n-acetyl-cystamine, and n-acetyl-cystamine-glycine) or indirect (2-oxothiazolidine-4-carboxylate) biochemical precursors, or their salts and esters, are reportedly used as medicinal agents and dietary supplements in treating various disease.sup.34,35,36,37,38.
GSH is also claimed to be useful as a chemoprotective agent that prevents neurotoxicity in cancer chemotherapy.sup.39 as well as in combination with antineoplastics in order to augment their effect.sup.40.
No reference, however, is currently available to GSSG as a medicine in its own right (sole substance) used to induce the endogenous production of cytokines and hemopoietic factors. The substance is known neither to have medicinal effects in human and animal diseases nor to be applied as a pharmaceutical agent for treating illnesses.